That's my thought too - honestly for a "young" actor - Matt Smith is learning how to play the world weary Doctor. Which I'm not sure I'm entirely crazy about. I'm sure they're trying to like make him Pertwee's second coming or something, give him more of a "I'm too old for this shit" kind of attitude versus Tennant's 80% of the time "Let's have fun!" or as his line from Waters of Mars when asked for name rank and purpose. "The Doctor. Doctor. Fun" 10 wanted to have fun. 11 seems overly pissed off at the universe - at least since the second part of Series 7 started. I mean I could see 10 doing that too - but then he'd get that little glint in his eyes and shout Allons-y! and run off and do something 'fun'.
Now... as for River's comment "seeing you so young" maybe she can recognize him as being 'younger' than her Doctor. After all, didn't 10 say he was around 950 years old and now I think they established by 7.2 that he's like 1100 - so he's still quite a bit older in 11 than he was in 10. I imagine even if she went back to the 1st Doctor's time she'd comment "My God you're young." even though he physically looks old enough to be her grandfather.
Although has anyone though that the reason why 11 was so moody and depressed - was because at that point, he sent River along to the Library? So he knew that it would be the last he'd really see of her at that point. That's why he's so hot for Clara Oswin Oswald. Rebound girl!